Silver halide photosensitive emulsions containing silver chloride are well known and offer many advantages. For example, they are more soluble than other photographically effective silver halides, so that shorter development and fixing times can be achieved.
In recent years, color photographic materials have become widely used and, in addition, development processing has come to be performed more and more simply and rapidly. On the other hand, high image quality and uniformity of finishing quality have been demanded.
In simplification and speeding up, specifically, reduction of the number of processing baths, reduction of the amount of replenishment (low replenishing), and shortening of processing times have come to be highly desirable in this industry. While silver chlorobromide emulsions have been found to be very advantageous in terms of rapid processing, their speeds are low, effective chemical and spectral sensitizations are difficult to obtain and the speeds which are obtained are generally unstable. These emulsions possess the further disadvantage of being easily fogged.
Several methods have been proposed to remedy the aforementioned disadvantages. There are known techniques such as those disclosed in JP-A-48-51627 (the term "JP-A" as used herein refers to a "published unexamined Japanese patent application") and JP-B-49-46932 (the term "JP-B" as used herein refers to an "examined Japanese patent publication") wherein water-soluble bromide ions or iodide ions are added after the addition of a photosensitizing dye to a silver halide emulsion. The method disclosed in JP-A-58-108533 and JP-A-60-222845 involves disposing on the grain surface a layer having 60 mol% or more of silver bromide, by adding bromide ions and silver ions simultaneously to silver halide grains having a high percentage content of silver chloride. Furthermore, there is a method of disposing wholly or partially a layer of 10 mol% to 50 mol% of silver bromide on the surface of these grains. JP-B-50-36978, JP-B-58-24772, U.S. Pat. No. 4,471,050 and West German Patent (OLS) 3,229,999 describe methods wherein polyphase structure grains such as core and shell, 2-fold structure grains or a composite structure grains are obtained by adding bromide ions, or simultaneously adding bromide ions and silver ions, to a silver halide having a high content of silver chloride, and by halogen conversion. The aforementioned patent documents describe techniques wherein local changes in the amount of silver bromide contained in individual silver chlorobromide grains (i.e., the inside or outside of the grains, or disposition on the grain surface) are provided, and by these means improved photographic properties are obtained.
Nevertheless, silver chlorobromide emulsions of sufficiently satisfactory gradation, pressure resistance and latent image preservation have not yet been obtained by these prior art methods.